This invention relates to a pneumatic tire having a unique tread configuration. The tire is particularly well-suited for use on the drive axle of vehicles that race on oval dirt tracks, such as Sprint cars.
Spring car racing is generally conducted on dirt tracks with a few races conducted on asphalt surfaces. The tracks are oval in shape and usually slightly banked. Ideally, race tires have a large amount of rubber in contact with the track. At most paved tracks, race tire treads are "slicks" which have no grooves. On dirt tracks, however, grooved tread patterns yield the best results.
Unlike a paved surface, dirt tracks can vary in consistency and surface conditions can change during a night of racing. The drivers prefer tires with soft rubber tread compounds on clay surfaced tracks and harder tread rubber compounds on the more abrasive racing surfaces. The tracks are watered down to keep dust to a minimum. As the track dries, the surface gets slick and hard. Under these conditions, drivers prefer hard compounds.
Sprint cars run a unique combination of tires. The front tires are the same from left side to right side. Typically the front tires have a wheel size of 8 inches in width.times.15 inches in diameter with the tread width of 8 inches. The front tires weigh about 15.5 pounds and are inflated to about 6-12 psi. The right rear tires have a wheel size of 18 inch width.times.15 inch diameter. The right rear tire has a tread width of about 17.5", weighs 33 pounds and is inflated to 6-8 psi. The rear left wheel is the stagger tire. The left wheel size is 14 inches width .times.15 inch diameter, weighs about 21.0 pounds and is inflated to 5-6 psi. The tread width is about 14.5 inches. Sprint cars are rear wheel drive vehicles. The rear tires provide forward traction while the front tires provide steering.
As illustrated in FIGS. 1 through 4, the prior art Sprint tire tread employed circumferential and lateral grooves having parallelogram shaped tread elements. At the tread edges, the lateral grooves were oriented at an angle .theta. of about 30.degree. relative to a plane passing through the axis of rotation, while the central tread elements were inclined at an angle .theta..sub.2 of about 10.degree. relative to the axis of rotation. The tires exhibited an overall net-to-gross ratio of about 71%.
It has been determined that by adjusting the tread pattern, an improved rear race tire has been developed that is well adopted for the hard slick dirt tracks.